The present invention relates to a reflective mask for extreme-ultraviolet (EUV) lithography, which comprises a reflective multilayer system on a substrate. Furthermore, the present invention relates to an EUV lithography apparatus having such a mask.
In lithographic structuring methods, for example of semiconductor components, the structure of a mask is projected onto an object to be structured using an EUV lithography apparatus. For this purpose, the mask is illuminated with the aid of an illumination system, and its structure is imaged onto the object to be structured with the aid of a projection system.
Light of an increasingly shorter wavelength is used so that increasingly fine structures can be produced in the manufacture of semiconductor components with lithographic methods. If light in the extreme ultraviolet (EUV) wavelength range is used, for example, at a wavelength of between about 5 nm and 20 nm, it is no longer possible to use lens-type elements in a transmission mode, but rather illumination and projection systems constructed of mirror elements each having reflective coatings adapted to the respective working wavelength are used. Masks must also be reflective. In contrast to mirrors in the visible and ultraviolet wavelength ranges, even theoretically only a maximum reflectivity of less than 80% can be achieved per mirror. Since EUV lithography apparatuses usually comprise a plurality of mirrors, each of them must have the highest possible reflectivity in order to ensure sufficiently high overall reflectivity.
Mirrors for the EUV wavelength range around 13 nm with high reflectivity values are known, for example, from DE 101 55 711 A1. The mirrors described there consist of a layer arrangement applied on a substrate, comprising a sequence of individual layers, wherein the layer arrangement comprises a plurality of partial layer systems, each having a periodic sequence of at least two individual layers of different materials forming a period, wherein the number of periods and the thickness of the periods of the individual partial systems decrease from the substrate to the surface. Such mirrors have a reflectivity of more than 30% at an interval of angles of incidence between 0° and 20°, wherein the reflectivity has strong variations in this interval of angles of incidence, however, which can lead to incorrect imaging in the context of an EUV lithography process.
The angle of incidence is defined as the angle between the incident direction of a light beam and the normal on the surface of the mirror at the point of incidence of the light beam on the mirror. The interval of angles of incidence can be derived from the angle interval between the largest and the smallest angle of incidence considered for each mirror.
Mirrors for the EUV wavelength range around 13 nm with high reflectivity values are also known from U.S. Pat. No. 7,474,733 B1. The mirrors shown there have high theoretical reflectivity values for multilayer systems, which consist of more than 30 identical periods of silicon and ruthenium layers for the highest reflectivity values. In practice, these theoretical reflectivity values cannot be achieved however, since intermediate layers are formed through interdiffusion between silicon and ruthenium layers, which leads to a loss of contrast at the layer interface and thus to a reduction in reflectivity. This also applies for the mirrors for EUV lithography known from U.S. Pat. No. 7,382,527 B2, wherein multilayer systems on the basis of silicon and ruthenium layers and multilayer systems on the basis of silicon and molybdenum layers are arranged on top of each other to achieve the highest possible reflectivities.
An EUV lithography apparatus with a mask is known, for example, from EP 1 434 093 A2.